A few questions:

• How will your program be notified (or detect) the external events?
• When an external event is notified, do you antisipate iterating through all your rules in order to decide what to execute?
• Alternatively, do you want the external event to cause only those rules that might cause some actions to be executed, to be evaluated?
• Where will your program pick up the value of the variables used within the conditions, from?
• How (and in what form) would your program pick up the rules?
  • Read them in from a file at startup?
  • Update them dynamically?
  • Import them as Perl code?
  • Parse as non-Perl expressions and interpret them at runtime?

There really isn't enough information in your post to base suggestions on. Things that come to mind are:

• triggers set on columns in a DB, that invoke stored procedures when values are added or updated.
• Something like a tied hash.

  When the main loop (or thread) monitoring the external events updates one of the keys in the hash, the tie implementation processes a set of rules and associated actions for that key.

Several other ideas are alluded to in my questions above, but there's little point in expanding them on the basis of what you've told us so far.